1. Field of the Invention
The present invention relates to a heart stimulator for electrical stimulation of the heart of the type having means for determining cardiac output and control means for controlling the stimulation in response to the determined cardiac output, and wherein cardiac output determining means includes means for measuring the pressure inside the right ventricle and producing a corresponding pressure signal, and means for producing from the pressure signal a control signal related to the cardiac output and supplying the control signal to the control means for controlling the delivered stimulation according to the control signal.
2. Description of the Prior Art
Several other manners of automatic adaption of pacemaker stimulation algorithms and parameters are also known. A common shortcoming to all these prior concepts is that no really appropriate criteria have been found for the algorithm and parameter optimization. Thus attempts have been made to e.g. optimize the AV-delay based on an assumed algorithmic relation between cardiac rate or cardiac activity and an optimal AV-delay. However, the results have been unsatisfactory.
Because of the wide variety of conditions affecting the needs of the patient, such as mental onset, nutrition, time of the day and season, diseases and individual peculiarities, it is not likely that an algorithmic relation will ever come close to the real situation. A system of this kind is unlikely to be effective unless the effectiveness can be measured and used as feedback in a stimulation control. Today pacemaker parameters are normally adjusted depending on diagnosis and the experience of the medical personnel.
The above mentioned U.S. Pat. No. 5,156,147 describes a rate adaptive pacemaker having a variable rate cardiac stimulating pulse generator and a sensor for monitoring some physiologic parameter for adjusting the pulse generator stimulation rate to meet physiologic demands. In addition thereto a hemodynamic sensor is operative to provide an output signal representing the pumping performance of the heart in response to the pacing stimulation. The hemodynamic monitoring sensor may measure the right ventricular pressure, and it is mentioned that the hemodynamic parameter used for controlling the pacing output is determined from the measured pressure signal.
The above subject is achieved in accordance with the principle of the present invention in a heart stimulator having a circuit for determining cardiac output and for producing a control signal corresponding to the cardiac output, a controller for controlling cardiac stimulation dependent on the control signal, and wherein the cardiac output determining circuit includes a pressure sensor which measures pressure in the right ventricle and which generates an electrical signal to corresponding to the measured pressure, an integrator supplied with the pressure signal which integrates the pressure signal between a start time and a stop time to produce an integration result, corresponding to the cardiac output, for use as the control signal, and a bandpass filter connected to the integrator and to the pressure sensor for filtering the pressure signal during a systolic phase to identify opening of a valve at the right ventricle as the aforementioned start time for the integration and to identify a closing of this valve as the aforementioned stop time.
Also U.S. Pat. No. 5,183,051 discloses a method for continuously determining cardiac output from measured arterial blood pressure data. Blood pressure is measured non-invasively or minimally invasively and the stroke volume is determined from the stroke area under the pulse pressure curve between the start of the systolic phase and the dicrotic notch, said stroke area being corrected for any surface portions related to reflected pressure waves.
The methods of determining cardiac output according to the two last mentioned publications are used for diagnostic purposes. Nothing is mentioned about using cardiac output as a parameter for controlling a heart stimulator.
The article K. Rohrbach, Ludwig-Maximilians-Universitat Munchen 1991, "Korrelation zwischen Schlagvolumen und abgeleiteten Druckparametern des rechten Herzens--Eine tierexperimentelle Studie-" discloses different methods of determining the stroke volume from a pressure signal obtained in the right ventricle of a pig. According to one method in this thesis is the pressure signal integrated during systole.